Upper-class twit and earl's daughter get married (6)
I believe the answer is:
united
'married' is the definition.
(similar in meaning)
'upper-class twit and earl's daughter' is the wordplay.
'upper-class' becomes 'u' (similar in meaning).
'twit' becomes 'nit' (both can mean a stupid person).
'and' is a charade indicator (letters next to each other).
'earl' becomes 'e'.
'daughter' becomes 'd' (genealogical abbreviation).
'u'+'nit'+'e'+'d'='UNITED'
'get' acts as a link.
(Other definitions for united that I've seen before include "collective" , "Agreed" , "Whole" , "See 1" , "Joined together as in alliance" .)